Furnaces for firing fossil fuels have long been employed to generate controlled heat, with the objective of doing useful work. The work might be in the form of direct work, as with kilns, or might be in the form of indirect work, as with steam generators for industrial or marine applications or for driving turbines that produce electric power.
Modern water-tube furnaces can generate large quantities of steam at higher pressures. Such modern water-tube furnaces for steam generation include central-station steam generators, industrial boilers, fluidized-bed boilers, and marine boilers. While, strictly speaking, the recuperative and conductive heat transfer system to which the present application is directed does not fall within any of these furnace type classifications, it might be considered to be more akin to a fluidized-bed boiler than to any of the other various types of modem water-tube furnaces referred to above. As such, the following discussion will include background relating to fluidized-bed boilers.
Fluidized-bed boilers have been used for decades to burn solid fuels with very high efficiency at a temperature low enough to avoid many of the problems of other solid fuel combustion techniques. As is well known to those of ordinary skill in the art, the word “fluidized” as employed in the term “fluidized-bed boiler” refers to the condition in which solid materials are given free-flowing fluid-like behavior. More particularly, as a gas is passed through a bed of solid particles, the flow of gas produces forces that tend to separate the particles from one another.
At low gas flows, the particles remain in contact with other solids and tend to resist movement. This condition is commonly referred to as a fixed bed. On the other hand, as the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed then becomes fluidized, that is, the gas cushion between the solids allows the particles to move freely, giving the bed a liquid-like characteristic. The state of fluidization in a fluid-bed-boiler combustor depends mainly on the bed-particle diameter and fluidizing velocity.
There are essentially two basic fluid-bed combustion systems, each operating in a different state of fluidization. One of these two basic fluid-bed combustion systems is characterized by the fact that at relatively low velocities and with coarse bed-particle sizes, the fluid bed is dense, with a uniform solids concentration, and has a well-defined surface. This system is most commonly referred to by those in the industry as a bubbling fluid bed, because the air in excess of that required to fluidize the bed passes through the bed in the form of bubbles. The bubbling fluid bed is further characterized by modest bed solids mixing rates, and relatively low solids entrainment in the flue gas. While little recycle of the entrained material to the bed is needed to maintain bed inventory, substantial recycle rates may be used to enhance performance.
The other of these two basic fluid-bed combustion systems is characterized by the fact that at higher velocities and with finer bed-particle size, the fluid bed surface becomes diffuse as solids entrainment increases, such that there is no longer a defined bed surface. Moreover, recycling of entrained material to the bed at high rates is required in order to maintain bed inventory. The bulk density of the bed decreases with increasing height in the combustor. A fluidized-bed with these characteristics is most commonly referred to those by those in the industry as a circulating fluid bed because of the high rate of material circulating from the combustor to the particle recycle system and back to the combustor. The circulating fluid bed is further characterized by very high solids-mixing rates.
Regardless of whether a bubbling type mode of operation is employed or whether a circulating fluidized bed type mode of operation is employed, there is a requirement that fluidizing air must be injected at a preselected velocity determined principally on whether the particular fluidized-bed is intended to operate in a bubbling bed type mode or in a circulating fluidized bed type mode.
Commonly, in large circulating fluidized bed boilers, the residual ash/sorbent particles and the flue gas, which are the byproduct of combustion of the circulating or bubbling fine solid fuel particles, are separated from each other, and the residual ash/sorbent particles are caused to flow to and through a fluid bed heat exchanger. No attempt is made to classify the type of or separate the residual ash/sorbent particles that are caused to flow back to and through the fluid bed heat exchanger. Rather, a mixture of all of the residual ash/sorbent particles that have been produced are caused to flow to and through the fluid bed heat exchanger.
In those implementations in which fluid bed ash coolers are employed to cool the residual ash/sorbent particles as these particles leave a large circulating fluidized bed unit, the fluid bed ash cooler may operate to separate large ash particles from the fines entrained therewith, before the separated fines are returned to the large circulating fluidized bed unit. However, the particles that are separated by the operation of such fluid bed ash coolers will include a mixture of all of the residual ash/sorbent particles that have been produced as a consequence of the combustion of the solid fuel in the presence of air. Furthermore, although there may be some separation of particles, here again no attempt is made to classify the types of particles that are included in the ash.
While it has been proposed to separate fluid bed particles including bauxite from a bubbling bed, this proposal did not suggest separating ash/sorbent particles from the fluid bed particles including bauxite before the later particles are caused to flow to the heat exchanger.
In summary, historically it has been the common practice in fluidized bed boilers, and in particular in large circulating fluidized bed boilers, not to classify/separate the various types of residual solid particles, before they are made to return to a fluid bed heat exchanger. In this regard, no attempt was made to effect a classification/separation between the types of solid particles, which collectively make up the residual ash/sorbent particles produced as a consequence of the combustion of the solid fuel in the presence of air in the combustor of fluidized bed units, either before or after such particles are caused to flow through a heat transfer system. Because of this, it was not historically possible to effect a complete decoupling of the combustion, heat transfer and environmental control processes in fluidized-bed boilers, and hence to separately control and/or optimize each of these processes.
However, recently a new and improved heat transfer system was developed that facilitates a complete decoupling of the combustion, heat transfer and environmental control processes in what is somewhat akin to a fluidized-bed type operation. This system is described in detail in U.S. Pat. No. 6,554,061, which shares inventors with those named in the present application and is assigned to the assignee of the present application.
As described in the '061 patent, the disclosed recuperative and conductive heat transfer system completely decouples the combustion, heat transfer and environmental control processes, thus allowing each of these processes to be separately optimized. In part, the patent discloses how (i) a moving bed of bauxite Al2O3 particles can be directed in a flow counter to the flow of the hot gases and any residual ash/sorbent particles from an internal or external heat source in one chamber, e.g. a combustion chamber, of the system to recoup heat, (ii) the heated bauxite Al2O3 particles can be separated from the hot gases and any residual ash/sorbent particles, (iii) the separated bauxite Al2O3 particles can transfer the recouped heat to a working fluid in another chamber, e.g. a plenum heat exchanger, and (iv) the separated residual ash/sorbent particles can transfer residual heat to preheat combustion air in another chamber, e.g. an air heater, and be subject to further combustion in the one chamber, e.g. a combustion chamber, of the system. While, as described in U.S. Pat. No. 6,554,061, the recuperative and conductive heat transfer system can be operated to maintain relatively low level emissions of the residual ash/sorbent particles, the patent does not address carbon dioxide, e.g., CO2, emissions.
In view of the ongoing debate over global warming has continued, the increasing attention being given to carbon dioxide (CO2) emissions from the burning of fossil fuels, and the expense and inefficiency of conventional techniques for capturing carbon dioxide emissions, a need exists for a recuperative and conductive heat transfer technique which reduces carbon dioxide emissions resulting from the burning of fossil fuels.